American Express, Visa, and MasterCard, will no longer allow their cards be used to purchase adult ads on Backpage.com, a website accused of facilitating sex trafficking of children. The move came after Chicago Sheriff Thomas J. Dart sent letters to the three companies asking them to “immediately cease and desist” from allowing the use of their cards for the site's adult listings.

“We shouldn’t have informal pressure from public officials forcing financial service companies into deciding which types of speech should and shouldn't be allowed,” he told USA Today.

Dart’s letters noted there are 20,000 ads posted on Backpage in the Chicago area each month and that each of the 800 times his office has responded to them, “We have made an arrest for crimes ranging from prostitution to child trafficking.” Cook County Sheriff’s spokesman Ben Breit told the Huffington Post in April alone, Backpage published 1.4 million adult ads in the United States.

Backpage isn’t the only online business Dart has targeted. In 2009, he sued Craigslist in an effort to have the popular site remove its adult services ads. When Craigslist closed its adult services section in 2010, much of the business moved to Backpage.

Backpage’s adult services business has largely prevailed in court and against legislative assaults. In 2012, Sen. Mark Kirk, R-Ill., introduced a bill calling for the elimination of the site’s “adult entertainment” section. In 2013, the National Association of Attorneys General asked Congress to amend the Communications Decency Act (CDA) so state and local governments could investigate and prosecute online classified-ad sites that promote prostitution and child sex trafficking. The American Civil Liberties Union called the effort “misguided,” and argued it would “cripple” free speech. The law remained unchanged. This past month, U.S. District Judge Richard Sterns dismissed a lawsuit filed in 2014 on behalf of three women sold for sex as children. The suit claimed the site knowingly supports, facilitates, and benefits from child sex trafficking in the United States.

After these efforts failed, Breit said the Cook County Sheriff's Office considered the implications if the major credit card companies simply said, “no more.”

“Everyone was kind of surprised no one had thought of it,” he said. “Everyone who has been working to fight Backpage, they’ve been, frankly, used to disappointment. They were waiting for the other shoe to drop—and it didn’t.”